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Farming Practices Influence Wild Pollinator Populations

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Farming Practices Influence Wild Pollinator Populations FIELD AND FORAGE CROPS Farming Practices Influence Wild Pollinator Populations on Squash and Pumpkin 1 2 3 RACHEL E. SHULER, T’AI H. ROULSTON, AND GRACE E. FARRIS Department of Environmental Sciences, University of Virginia, Charlottesville, VA 22904-4123 J. Econ. Entomol. 98(3): 790Ð795 (2005) ABSTRACT Recent declines in managed honey bee, Apis mellifera L., colonies have increased interest in the current and potential contribution of wild bee populations to the pollination of agricultural crops. Because wild bees often live in agricultural Þelds, their population density and contribution to crop pollination may be inßuenced by farming practices, especially those used to reduce the populations of other insects. We took a census of pollinators of squash and pumpkin at 25 farms in Virginia, West Virginia, and Maryland to see whether pollinator abundance was related to farming practices. The main pollinators were Peponapis pruinosa Say; honey bees, and bumble bees (Bombus spp.). The squash bee was the most abundant pollinator on squash and pumpkin, occurring at 23 of 25 farms in population densities that were commonly several times higher than that of other pollinators. Squash bee density was related to tillage practices: no-tillage farms hosted three times as great a density of squash bees as tilled farms. Pollinator density was not related to pesticide use. Honey bee density on squash and pumpkin was not related to the presence of managed honey bee colonies on farms. Farms with colonies did not have more honey bees per ßower than farms that did not keep honey bees, probably reßecting the lack of afÞnity of honey bees for these crops. Future research should examine the economic impacts of managing farms in ways that promote pollinators, particularly pollinators of crops that are not well served by managed honey bee colonies. KEY WORDS pollination, squash, pumpkin, sustainable agriculture, tillage INSECTS ARE OFTEN VIEWED as the scourge of agriculture, 1998). The prospect of future honey bee shortages has yet many food crops require insect pollination to set led to a recent interest in the role of wild pollinators fruit. Thus, farm management practices must attempt in agricultural systems (Allen-Wardell et al. 1998, Kre- to reduce the negative effects of herbivorous or dis- men and Ricketts 2000, Westerkamp and Gottsberger ease-transmitting insects while maintaining an envi- 2000, Kremen et al. 2002). Because wild pollinators ronment conducive to pollinator activity. The honey generally cannot be introduced suddenly to agricul- bee, Apis mellifera L., is the predominant managed tural systems in adequate numbers to ensure pollina- pollinator in much of the world (Robinson et al. 1989). tion, successful management approaches are likely to Because it occurs in very large colonies, visits many focus on managing farm conditions rather than the different crops, and can be transported into and out of pollinators themselves. agricultural Þelds, it has provided agriculture with the Pioneering work by Kremen et al. (2002) has shown ability to take aggressive insect control measures that wild bee populations vary with farming practices through much of the growing season without suffering and the distance from farms to natural habitats. Work- substantial losses of insect pollination. ing in a major agricultural area of California, they The number of managed honey bee colonies in the showed that organic farms near natural habitats hosted United States has recently declined due to difÞculties sufÞcient wild bees to provide full pollination services in managing them. These difÞculties include the re- for watermelon (Citrullus spp. Shrad.), a lucrative cent establishment of parasitic mites and hybridiza- crop with large pollination requirements. Wild bee tion with the Africanized honey bee, Apis mellifera populations were diminished at all other farms, and scutellata (Ruttner), in some regions (Peng and Nasr full pollination required the addition of honey bees. 1985, Weinberg and Madel 1985, Allen-Wardell et al. This work points to the possibility that farm manage- ment practices that encourage wild pollinator popu- 1 Department of Biology, Oberlin College, Oberlin, OH 44074. lations may provide ensurance against pollination 2 Corresponding author: University of Virginia, Blandy Experimen- losses incurred by further honey bee declines and tal Farm, 400 Blandy Farm Lane, Boyce, VI 22620 (e-mail: reduce costs associated with renting or maintaining [email protected]). 3 Department of Hispanic Studies, Brown University, Providence, honey bee colonies when they are unnecessary. Some RI 02903. historical evidence shows that honey bees became 0022-0493/05/0790Ð0795$04.00/0 ᭧ 2005 Entomological Society of America June 2005 SHULER ET AL.: SQUASH AND PUMPKIN POLLINATION 791 Fig. 1. Distribution of 25 participating farms that grew squash, pumpkin, or both. increasingly necessary in North American agriculture pumpkin is the bee Peponapis pruinosa Say, a special- when intensive farming practices reduced the popu- ized, widespread pollinator that collects pollen only lations of wild local bee populations (Batra 1995). from the genus Cucurbita (Hurd et al. 1974). Because To sustain populations, bee species require food there are no wild Cucurbita in this region, the P. resources throughout their active period and undis- pruinosa population is entirely dependent on culti- turbed nesting substrate during their developmental vated Cucurbita and cannot maintain refuge popula- period. Food resources comprise pollen and nectar, tions far from agricultural areas. Our work provides an which together provide the protein, carbohydrates, indication of which bees are primarily responsible for and micronutrients required for larval development pollination of squash and pumpkin in this region and and adult maintenance (Michener 2000). Nesting sub- which farming practices may have the greatest inßu- strates vary, but most bee species are either cavity ence on their pollinator populations. We focus on nesters that occupy existing structures such as hollow farming practices that seem most directly related to plant stems (Frankie et al. 1998) or ground nesters that the life cycle of wild bees: tillage (survival of immature excavate tunnel systems in earthen banks or bare bees), crop diversity (continual food supply), and patches of soil (Chapman et al. 1990). Natural cavities pesticide use (direct impact on adults). This study has are most likely to occur outside the planting area, but implications both for the economics of agriculture and bare earth occurs commonly within Þelds, and many the conservation of biodiversity in agroecosystems. bee species nest alongside crops (Mathewson 1968). The survival of offspring within planting areas de- pends on nests not being disturbed during develop- Materials and Methods ment, which takes only a few weeks during the sum- mer in species that have multiple generations Participating Farms. We compiled a study group of (multivoltine) but takes most of the year for univol- 25 farms within an Ϸ100 by 130-km area of Virginia, tine species and for the overwintering generation of West Virginia, and Maryland (Fig. 1). Participating multivoltine species. farmers were initially contacted at regional farmersÕ In the current study, we examine the effect of farm- markets or at their own farms by driving through the ing practices on pollinator populations of cultivated countryside looking for large plantings of squash and squash and pumpkin in the tristate border area of pumpkin. Farmers were interviewed concerning their Virginia, West Virginia, and Maryland. Squash and management practices, including pesticide use, tillage, pumpkin (both in genus Cucurbita) are valuable, com- diversity of crops grown, use of managed honey bee monly grown crops that require insects for pollination. colonies, and the number of consecutive years that Although honey bee colonies are often placed in squash or pumpkin had been grown on the site. squash and pumpkin Þelds for pollination, honey bees Twelve of the participating farms did not use pesti- prefer other crops, weeds, and wild plant species and cides, whereas 13 applied one or more types of pes- often fail to visit the target plants if other options are ticides. Planting area of the target crops ranged from available (Delaplane and Mayer 2000). One of the Ͻ0.5 to 40 ha (median 0.8). Total farm area ranged most effective and persistent pollinators of squash and from Ͻ0.5 to 400 ha (median 80.9). 792 JOURNAL OF ECONOMIC ENTOMOLOGY Vol. 98, no. 3 Insect Surveys. Each farm was surveyed for insect 0.5 visitation during 1 d between 7 July and 5 August 2003. p = 0.006 Surveys were limited to sunny-to-moderately cloudy 0.4 days in the morning. Squash and pumpkin ßowers are open from predawn until Ϸ1000 hours in this part of the United States, and individual ßowers last a single 0.3 day only. Although squash bees ßy from predawn until ßower closure, other potentially important pollinators 0.2 such as honey bees and bumble bees (Bombus spp.) were not active until well after dawn. Thus, we con- Þned insect surveys to a period from 0730 to 0900 0.1 hours (EST) to make sure that we would encounter all of Bees Per FlowerDensity the main pollinator species if they were present on the 0.0 target plant species at the study site. Pollinator species Honey Bees Bumble Bees Squash Bees that may show little activity before 0900 hours, such as sweat bees (Halictidae), are likely underestimated by Fig. 2. Density of three main squash and pumpkin pol- our methodology. Unless there is a shortage of polli- linators across 25 farms. Mean and SE given for each taxon nators, however, most of the pollinating activity has across all sites at which the taxon occurred (i.e., excluding zero values). already been carried out by that time of day. Surveys were carried out by one to three researchers trained to recognize the main pollinators without collection. Z-statistic as the original data ϩ 1 divided by the total Training was done through Þeld experience with an number of permutations ϩ 1.
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